As we walked into the wood this weekend, it seemed the whole of the canopy must have come down to the floor with a thump. Suddenly, there was barely a leaf left on a tree, and the woodland floor was a thick, dense mat of sound absorbing, wet foliage. Paths, rocks, dead wood were all obscured and the result was a little disorientating. Only the ground beneath the yews remained clear of the autumn’s carpet; here the limestone blocks on which our wood stands were exposed, white angular chunks balanced on a crazy paving of moss and lichen encrusted slabs.
We stamp over the woodland floor, most of the time without giving it a thought. Sometimes, as a path becomes particularly well-trodden, a piece of almost sculptural rock appears white through the soil. In places, the limestone is at the surface and you have to concentrate as you walk to avoid a boot or ankle becoming caught in a rocky fissure. But other than that we don’t often dwell on the fact that our wood has a very special floor, a terrain found in few places round the world because of the very specialised circumstances required for its formation. Our wood has grown, at least in part, on a limestone pavement; it has a story of its own, one that stretches back far into the past.
Around 330 million years ago, the floor of our wood was beneath the sea, a lovely, warm, shallow Carboniferous sea. The shells of dead sea-creatures drifted with other sediments, gradually settling onto the sea bed. Over geological time, these sediments were compressed to become layers of rock, rich in fossils, with the different layers representing different rates of sedimentary deposition. These are the limestones of our woodland, and we often come across fossils as we are exploring. Mostly these fossils are of corals, but we do find shells too.
The story does not end there. Next our limestone floor was shifted by the volcanic activity in the area, unimaginable forces lifting it up, breaking it into sections divided by vertical cracks. Then followed the ice age. Immense glaciers, grinding rivers of frozen water, scraped away everything from the surface of the limestone, scouring the rock as it passed above so that when the ice melted, erosion by rain over thousands of years could shape the limestone in a unique way. Limestone is a hard rock, but it is made of calcium carbonate. Water falling as rain has carbon dioxide dissolved within it; it is a very dilute carbonic acid. Acid will dissolve calcium carbonate and this is what happened to our limestone. The rain weathered the scoured limestone, falling onto its surface and running down through the cracks, slowly dissolving away the surface.
The cracks widened, eventually producing the block work which is the diagnostic feature of a limestone pavement, (technically the blocks are called clints and the gaps between are called grykes).
Where limestone pavements are lightly grazed, they remain open with highly specialised plant communities developing in the grykes – these shady places are ideal for mosses and ferns. Trees such as ash and yew are also able to grow up from the grykes, but because of a lack of nutrients and water (any rain drains away down the grykes very quickly) these trees may only manage to grow a few millimetres a year so are very stunted. Where limestone pavement it is not grazed, and soil begins to accumulate, woodland will develop; this is what has happened in our woodland.
Because of the tree and soil cover, it’s difficult for us to tell exactly how much of our limestone is actual limestone pavement, though a good proportion is at least. We are very lucky to still have it – in the past limestone pavement was ripped up for walls, buildings and, especially in Victorian times, ornamental rockerys. Now all surviving limestone pavement is protected by law and we feel very privileged to be caring for a small piece of this distinctive geological feature.
So as we walk round the wood, it’s good sometimes just to remember how unique our woodland floor is, to appreciate the special sequence of events necessary to create it. And there is one last intriguing thing. On the lowest level of the woodland, ten metres or so from the track as it enters the wood, is a relatively new, perfectly round, two metre deep, steep sided hole looking for all the world like a sink hole. Everyone we ask about it can come up with no other explanation. We have a wonderful limestone pavement – is it possible we have other elements of limestone landscape, an underground cave or water system, for example, which has partly collapsed to create this sink hole? Who knows? It is certainly something we can have fun thinking about and admiring – though until we are clearer about what exactly it is, it’s something we like to enjoy from a little distance, until we are absolutely sure we are on solid ground…